Nanotechnology-Enabled Sensors

92 Chapter 3: Transduction Platforms
Polymeric membranes: They are used for measuring ions such as potassium,
calcium, fluoroborate, nitrate, perchlorate, and even other applications
such as water hardness. Polymer membrane electrodes generally
consist of various ion-exchange materials incorporated into an inert matrix
of polymer such as polyethylene, PVC, polyurethane and silicone.
Gas permeable membrane: Sensing electrodes are available for the measurement
of gas species such as ammonia, carbon dioxide, dissolved oxygen,
nitrogen oxides, sulfur dioxide and chlorine gas. These electrodes
have a gas permeable membrane, selectively filtering target analyte gases.
Currently electrochemical sensors are widely used for determining the
oxygen content of gas species for industrial applications especially in the
automotive industry. They are particularly utilized to analyze exhaust
gases in a combustion process as they have relatively short response time
in comparison with other gas sensors. 11
Zirconium oxide (zirconia) electrochemical oxygen sensor is one of the
most common gas sensors. In such a sensor, the electrolyte is typically
made of zirconium oxide (Fig. 3.17). The sensor electrodes can be made of
platinum which also operates as a catalyst.
Fig. 3.17 A schematic of zirconia oxygen sensor.
This sensor operates at elevated temperatures of higher than 400°C.
Based on the partial pressure of oxygen, the porous zirconium allows the
movement of oxygen ions from a higher concentration to a lower concentartion.
The diffusion of oxygen ions produces a voltage across the device,